The present invention relates to an apparatus for triggering passenger protection systems, particularly restraining systems in vehicles on at impact. Such systems are known e.g. from the DE-PS 26 12 215. The triggering of such systems at the appropriate point in time presents certain difficulties. On one hand, because the rapidity of the sequences of movement, the sensitivity of the sensor must be great enough so that, e.g. during an impact at 50 km/hour, the registering of the impact, as well as the safety measure itself, such as tightening of the straps or blowing up of cushions, must be completed within 40 ms. On the other hand, triggering may not be effected for obvious reasons when criteria which only resemble an impact are present at the sensor, as can occur, for example, when driving over an obstacle or even during normal braking of the vehicle. Moreover, it must be taken into account that below an acceleration of approximately--4g the passengers can support themselves without additional restraining measures, i.e. the triggering of safety device can result in increased risk in this event, resulting in the opposite of the intended purpose. It has been shown that a dangerous impact situation is characterized above all in that the polarity of the acceleration does not change over a distinctly longer period of time in contrast to intensive short-term accelerations, as occur e.g. when driving over a curb or also as a result of a blow by a hammer. The acceleration changes repeatedly within a very short period of time.
In order to separate critical impact situations from uncritical impact situations in an improved manner, it is known, e.g. from the US-PS 37 01 903, to provide an amplitude limiter, an integrator, and a threshold circuit in addition to the acceleration receiver. It is the object of the amplitude limiter to limit upwardly the amplitude of an impact, so that the following integrator reaches the threshold voltage of the threshold value switch, which is connected downstream, only during impacts at intense, more sustained accelerations. If the amplitude of impacts is too small, i.e. if it lies below the level fixed by the amplitude limiter, no output signal appears at the input of the integrator. In addition, the integrator causes the triggering voltage for the threshold value switch connected downstream to be reached only after the expiration of a predetermined period of time.
A disadvantage of the previously known systems consists in that they do not trigger or trigger late at low accelerations such as occur e.g. during oblique impact.
Instead of using an amplitude limiter, it is further suggested to deduct a constant value from the output value of the acceleration receiver. Since the integrator is adjusted in such a way that it is not capable of transmitting a negative output signal, output signals of the acceleration receiver lying below the threshold voltage have no effect on the output of the integrator.
European patent application EP-A-O 156,930 discloses an apparatus for triggering a restraining system of a motor vehicle upon impact in which an output signal from an acceleration receiver is communicated to an integrator the output signal of which actuates the restraining system as soon as the integrator output signal exceeds the triggering threshold value. A variable integration threshold value is applied to the integrator, and a reference value of the integrator is shifted as a result of the differentiation of the output signal of the threshold value switch. First, the integration threshold value is changed in accordance with a change in the triggering threshold value in order to insure a reliable response and a reliable triggering of the restraining system in response to short input impulses. However, this triggering system does not permit to obtain an early reliable triggering of the passenger protection system at comparatively low acceleration values which occur at an oblique impact.
The aforementioned systems do indeed solve the problem of only registering accelerations which lie above a determined level and a determined time period. But it is a disadvantage in these systems that they do not trigger or trigger late at low accelerations such as occur during an oblique impact, since, due to the deduction of a constant value from the measured acceleration signal, the acceleration value remaining to be integrated leads too late or not at all to an integration value which exceeds the triggering threshold of the system connected downstream of the integrator. Therefore, the present invention is based on an apparatus for triggering passenger protection systems, particularly restraining systems, in vehicles during impact, in which the output signal of an acceleration receiver which is reduced by an amount corresponding to an integration threshold value, or a signal derived from this output signal, is integrated in an integrator, wherein the output signal of the integrator triggers the passenger protection system, possibly via an output stage, as soon as the integrator output signal exceeds a triggering threshold value.
It is the object of the invention to provide measures in such an apparatus which ensure a prompt, secure triggering of the passenger protection system during critical impact processes at comparatively low acceleration values.